The present invention relates to improvements in the rod-in-tube method (hereinafter referred to as "RT method") of fabricating the preforms of optical fibers from silicate glass.
Optical fibers are produced usually by preparing a preform, and heating and drawing the preform. The characteristics of optical fibers are almost dependent on the characteristics of the preform unless the preform is drawn by an improper technique.
Generally known as methods of fabricating optical fiber preforms are the chemical vapor deposition method (CVD method) and the RT method. As disclosed in Japanese Patent Publication (Tokkyo Kokoku) No. 29953/1976, the CVD method comprises the steps of depositing a thin cladding layer on the inner surface of a silicate glass tube, depositing on the cladding layer a thin core layer having a higher index of refraction than that of the cladding layer, and collapsing the resulting tube by heating to a solid preform free from any interior space. Presently this method is most widely used for the preparation of optical fiber preforms and has the advantage of being capable of fabricating low-loss optical fibers. The CVD method, however, requires repetition of the vapor deposition step a large number of times, for example, 50 to 100 times, and is therefore low in production speed. Especially if it is desired to obtain preforms for giving optical fibers with outstanding transmission characteristics and high stability, it is difficult to provide an increased amount of deposition at a time for forming the core layer. Thus the method involves limitations on the size of preforms and on the length of fibers available. Since increased optical losses will result from connection of optical fibers if they are low in dimensional accuracy, for example, the core is eccentric relative to the cladding or has low circularity, the CVD method must be practiced under strictly controlled conditions to overcome the difficulties encountered in producing preforms with increased dimensional accuracy, namely with sufficiently high core circularity and reduce core eccentricity. For these reasons, the CVD method still remains to be improved in its amenability to the quantity production of preforms, yield and accordingly manufacturing cost.
According to the RT method which is known for a long time, a glass rod serving as a core is inserted into a glass tube useful as a cladding, and the assembly is heated to a high temperature so that the tube is collapsed to heat-adhere to the rod, yielding an optical fiber preform. Since the rod having accurate dimensions and the tube are merely thus heat-adhered into a preform, it is easy with the RT method to obtain preforms in large sizes, with high dimensional accuracy and relatively free of the problems, such as core eccentricity and low core circularity, which are inherent in the CVD method. With respect to the dimensional accuracy, therefore, the method affords products in increased yields with reduced product-to-product or lot-to-lot dimensional variations even when performed for mass production. However, the RT method has a serious drawback. It is difficult to prepare preforms in which the interface between the rod and the tube is free from irregularities such as voids and foreign matters. Those irregularities, especially voids, would cause light scattering losses in the resulting optical fiber. This leads to difficulty in producing low-loss optical fibers which are comparable to those obtained by the CVD method.
Whereas research has been directed also to the RT method in an attempt to provide preforms free from imperfections at the interface between the rod and the tube, none of the proposals heretofore made have proved fully satisfactory in the reduction of losses as will be described below.
For example, it has been proposed to clean the surfaces of the rod and the tube with hydrofluoric acid, hot hydrogen fluoride gas or a mixture of hydrogen chloride and helium before the rod and the tube are heat-adhered together, but this method is unable to remove extraneous matter such as carbon particles and is ineffective for diminishing scattering losses due to the presence of irregularities at the interface between the core and the tube. Additionally when hydrogen fluoride gas or like hydrogen-containing gas is used at a high temperature, OH group derived from the hydrogen contained in the gas will be incorporated in various forms into the rod and the tube, entailing an increased absorption loss.
It has also been proposed to pass oxygen gas through the clearance between a rod and a tube maintained at a high temperature of about 1,500.degree. C. to remove foreign matter from the opposed surfaces of the rod and the tube by decomposing and/or oxidizing the foreign matter and thereafter heat-adhering the rod and the tube. With this method, the foreign matter or the resulting oxide, when having a boiling point higher than the above high temperature, will not be thoroughly removed but remain between the rod and the tube. The method is also ineffective for sufficiently reducing the scattering loss attributable to interface irregularities such as voids.
Thus it has been thought that despite various attempts, the RT method has difficulties in producing low-loss optical fibers such as those prepared by the CVD method although having the advantage of affording optical fiber preforms in large sizes with high dimensional accuracy.